1. Field of the Invention
The invention relates to minimizing and/or damping disturbances that can affect a spin-stabilized satellite, for example ,a geostationary remote sensing satellite.
It is directed in particular, although not exclusively, to minimizing and/or damping nutation phenomena.
2. Description of the Prior Art
It is usual to distinguish two phases (of very different duration) in the service life of a satellite: the orbital injection phase in which the satellite is moved to its service orbit from a transfer orbit and placed in an appropriate attitude, followed by the station-keeping phase, i.e. the operational life of the satellite, i.e. the phase during which the satellite fulfills its mission.
At present, the orbital injection phase for spin-stabilized satellites uses a Solid (powder) fuel apogee motor; it is intended that future generations of satellites will use a liquid fuel motor, having several nozzles distributed around the rotation axis (for example, see European Patent-A-0 445 011). This enables the use of a unified fuel supply system for the apogee motor and for the attitude control thrusters; it also increases operational flexibility as liquid fuel thrusters have an advantage over solid fuel thrusters in that they can be turned on and off at will. More generally, liquid fuel thrusters add to the flexibility of satellite design. It follows that there are substantial mass saving possibilities.
As mentioned in document European Patent-A-0 445 011, it is tolerable for there to be only one eccentric thruster in service, given the gyroscopic stiffness of the spin-stabilized satellite; however, carrying out orbital injection using a single eccentric thruster can cause disturbances including nutation and variation in the spin speed of the satellite, which may require costly speed adjustments at the end of orbital injection.
After orbital injection the satellite may suffer various disturbances likely to degrade its performance in fulfilling its mission (its remote sensing mission, for example):
movement of mobile parts of the satellite (for example scanning mirrors and/or mechanisms) parallel to the rotation axis can cause nutation of the satellite and variation of its inertial properties, PA1 operation of small thrusters to perform control maneuvers (attitude, orbit, spin speed) also causes nutation, PA1 solar eclipses, with a duration up to 72 minutes on 90 days in each year, eliminate the possibility of synchronizing EAST-WEST scanning (imaging) operations with the Sun, and cause a significant variation in the spin speed of the satellite because of thermal contraction of the satellite. PA1 a nutation characterizing step in which the actuator is activated in a predetermined mode and the resulting period of nutation is measured, this characterization step having a duration equal to an integer number of the nutation periods, PA1 a preliminary step during parts at least of which the actuator is activated in the predetermined mode, the duration of this preliminary step being equal to a predetermined fraction of the nutation period, this preliminary step being adapted to bring the satellite into a configuration in which triggering of the service mode of operation induces minimal nutation, PA1 an operational step during which the actuator is activated in the service mode of operation for any predetermined duration, and PA1 a final step, the duration of which is equal to that of the preliminary step and which is symmetrical thereto about the operational step. PA1 the actuator is an on-off device, the predetermined mode is identical to the service mode of operation, the preliminary step constitutes a first sub-phase having a duration of 1/6 nutation period during which the actuator is activated and a second sub-phase of the same duration during which the actuator is de-activated, the final step constituting a first sub-phase with a duration of 1/6 period during which the actuator is de-activated and a second sub-phase during which the actuator is active in the service mode of operation, PA1 the on/off actuator is a thruster eccentric to the spin axis, PA1 the actuator is a progressive action device, the predetermined mode corresponds to half the service mode of operation, and the preliminary and final steps each have a duration of one half-period, the predetermined mode being maintained throughout the duration of the preliminary and final steps, PA1 the actuator includes a member mobile substantially parallel to the spin axis, the predetermined mode being a displacement of this mobile member at half its speed of displacement in the service mode of operation, and PA1 the nutation period is measured by accelerometers with sensing axes substantially parallel to the spin axis.
With regard to the first two disturbances, the strategy currently adopted is to damp the induced nutation of the satellite by means of passive dampers (typically of the mercury or "Flutex" type); this does not prevent degraded imaging during and just after a maneuver; also, with regard to the first disturbance, this strategy may be unsuitable for future satellites in which the mobile parts are larger (heavier instruments), faster (shorter imaging cycles) and move over greater distances (black body pointing for infrared sensor calibration).
With regard to the third disturbance, the strategy currently adopted is to synchronize imaging by means of the terrestrial sensor or the on-board clock; in both cases the image is degraded (high noise in the case of the terrestrial sensor, fast drift in the case of the on-board clock); this is deemed to be tolerable in that the visible light channels cannot be used anyway during a solar eclipse; on the other hand, on second generation satellites, the provision of many infrared channels which can continue to operate even in a solar eclipse means that this problem has to be reconsidered.
A first object of the invention is to minimize the effects of nutation induced by the prolonged operation of an actuator operative parallel to the rotation axis or by displacement of mobile parts parallel to this axis.
Another object of the invention, which may be independent of the first object, is to provide effective compensation of pre-existing nutation by means of appropriate transverse pulses.
A further object of the invention, which may be independent of the previous two objects, is to minimize the aforementioned effect of spin speed variation when thrust is applied by a single eccentric thruster.
A still further object of the invention, independent of the previous three objects, is to maintain synchronism between imaging and rotation, even during a solar eclipse.